The present invention relates to the regeneration of Fischer-Tropsch (F-T) catalysts, in particular, the finely divided solid catalyst used when the F-T reaction is carried out in slurry reactors.
The F-T synthesis is carried out in various types of slurry reactors, one example being bubble column reactors. The operation of bubble column slurry reactors is flexible. They combine low diffusional resistance with efficient heat transfer. Examples of bubble column reactors used for F-T synthesis are shown in the present Applicants' WO 93/16796, WO 93/16795, WO 94/16807 and British Patent Application No. 9317605.5. Mechanically agitated slurry reactors are particularly convenient for laboratory studies due to the low mass-transfer and heat resistance. These features make them suitable for the determination of reaction kinetics. However, this technique is generally of minor importance on an industrial scale.
The reaction products are a complicated mixture, but the main reaction can be illustrated by the following equation: EQU nCO+2nH.sub.2 .fwdarw.(--CH.sub.2 --).sub.n +nH.sub.2 O (I)
where (--CH.sub.2 --) represents a straight chain hydrocarbon of carbon number n. The carbon number refers to the number of carbon atoms making up the main skeleton of the molecule. In F-T synthesis, the products are generally either paraffins, olefins, or alcohols. Products range in carbon number from 1 to 50 or higher.
The common F-T catalysts are nickel, cobalt, and iron. Nickel was probably the first substance to be recognised as capable of catalysing the reaction of syngas to hydrocarbons, producing mainly methane. Iron and cobalt are able to produce higher chain hydrocarbons and are, thus, preferred as catalysts for the production of liquid hydrocarbons. However, other metals are also capable of catalysing the conversion of syngas.
In recent years considerable advances have been made in the development of F-T catalysts and much of this work is well documented in the art. As far as slurry reactors are concerned, suitable catalysts are disclosed in EP-A-313375 and in the present Applicants' EP-A-404902. The latter reference describes broadly a cobalt catalyst which is composited on an alumina support together with rhodium, and platinum and/or iridium. The production of hydrocarbons by the F-T synthesis method, using cobalt-based catalysts can result in catalyst deactivation. This may be caused by various undesirable phenomena, such as re-oxidation of the active metal.
There are several known regeneration methods. These generally fall into two categories, namely, oxidative and reductive. In oxidative regeneration, the catalyst is removed from the slurry environment after which a treatment with air or oxygen is carried out to burn away the hydrocarbon rests. This is followed by normal catalyst activation. In reductive regeneration, the catalyst can be activated in-situ using hydrogen. Different systems can be used to achieve this, such as are described in U.S. Pat. Nos. 5,260,239 and 5,268,344, for regeneration in a slurry reactor.
In U.S. Pat. No. 5,260,239 a system is described in which there is a reaction vessel and a regeneration vessel. There is a mutual exchange of the catalyst slurry between the vessels by way of downcomers and H.sub.2 gas is injected into the rejuvenation vessel to regenerate the catalyst.
In U.S. Pat. No. 5,268,344, the reactor vessel itself contains vertical tubes open at both ends. H.sub.2 gas is injected into the bottom of the tubes which has the effect of drawing slurry up the tubes and while in the tubes, regenerating the catalyst.
These arrangements all suffer the drawback that a separate source of H.sub.2 gas must be provided to achieve regeneration.